History

What do you mean by "I have the same roll more than once"? Is it the same directory on the hard disk that is present more than once in darktable or have you several directories containing the same photos and have imported them all?

In the latter case just select all images in the filmroll (ctrl-a) and remove them (del). Once there are no more images in the filmroll it will be removed automatically.

If darktable however has the same folder more than once in its database then you have found a bug. A really strange one.